The invention relates to devices, such as flight simulators where the position (angular or linear) of a movable element relative to a fixed element must be measured very accurately. The invention relates more specifically to the servo control of such devices.
A flight simulator provides a platform supported in a gimballed arrangement, so that the platform may be rotated about one of the axes. A control system for rotation about one of the axes typically includes an actuator such as a torquer, a position sensor such as a resolver, a mechanism for generating position commands, and a closed loop servo for causing the position to track the command.
U.S Pat. No. 4,253,051, hereby incorporated by reference, describes portions of a representative prior art system. In brief, the position sensor typically includes a stator, a rotor inductively coupled to the stator, and circuitry for exciting the stator and detecting the induced signal in the rotor. The relative phase between the stator and rotor signals is representative of the relative displacement between the stator and rotor.
A known way of measuring the phase difference is to generate pulses at a frequency that is a fixed factor multiple of the rotor output signal frequency. The number of pulses occurring between corresponding points in the stator and rotor signal cycles, divided by the fixed factor, gives the fraction of a cycle that the two signals are out of phase. The frequency multiplication may be accomplished by a digitally closed phase locked loop (sometimes referred to as a PLL or a loop). The PLL includes, among other things, a frequency divider corresponding to the fixed factor multiple to be achieved, and a counter whose output represents the relative position.
It is known in the art to provide separate coarse and fine measurements of the relative position. Each requires a separate resolver and PLL. The coarse number is capable of defining the position within the entire expected range, while the fine number defines positions within a range that is much narrower. For example, a two-pole resolver generates a 360.degree. phase shift between the stator excitation signal and the rotor output signal for every 360.degree. of relative mechanical rotation, and is suitable for extracting the coarse number. A 720-pole resolver produces an electrical phase shift of 360.degree. for every 1.degree. of relative mechanical rotation, and may be used to extract the fine number.
The divisions are typically chosen so that the coarse and fine numbers have a common range of resolution, i.e., the least significant portion or digit of the coarse number would overlap the most significant portion or digit of the fine number. Using a coarse number divisor of 3600 and a fine number divisor of 10,000 gives a coarse number that is generally accurate to 0.1.degree. over a range of 0-359.9.degree. and a fine number that is accurate to 0.0001.degree. over a range of 0-0.999.degree.. Thus, each number has a tenths digit. In a typical system, the tenths digit of the coarse number will deviate from the tenths digit of the fine number as the rotors are rotated over the full 360.degree. range, with a typical error being about .+-.0.2.degree..
The coarse and fine numbers are used to generate coarse and fine position errors for the servo as follows. The output from the coarse (or fine) PLL counter is applied to one side of a digital comparator while the coarse (or fine) angle command is applied to the other. Once the coarse (or fine) PLL counter data corresponds to the coarse (or fine) command, a pulse is generated which is applied to one input of a digital phase detector, while the reference clock (zero degree strobe, or ZDS) is applied to the other input. The output from the phase detector is a pulse train whose pulses each have a width proportional to the difference between the actual coarse (or fine) position number and the coarse (or fine) position command. This pulse train is then applied to a pulse-width to DC voltage converter, resulting in an analog error voltage proportional to the coarse (or fine) position error.
The coarse and fine position error voltages are applied to the two poles of an analog switch. Since it is the intent to position the relatively movable objects to within the resolution of the fine position command in a closed loop servo system, it is necessary to select between the coarse and fine position errors. This is done by using an analog voltage window comparator. For coarse position errors greater than the comparator's threshold voltage, the coarse position error is selected. When the coarse position error voltage falls below the threshold, the comparator output changes and the fine position error voltage is selected.
The basic philosophy behind generating separate coarse and fine position error voltages has been used on several systems with little or no difficulty. Precision rate-of-turn tables and precision angular positioners are well suited to this. However, for systems in which high dynamic motion and/or large amplitude motion is expected, the transition between coarse and fine loop control may result in less than smooth motion.